In this case, we are setting the MaxDropDownItemCount property to 3. When the AutoCompleteDropDown popup is shown, only 2 items are completely visible. The Height of the dropdown is not correctly calculated and does not respect the size of the sizing grip area which is 20 px in this theme.

As a workaround, we can manually change the height in the PopupOpened event:

this.radTextBoxControl1.TextBoxElement.AutoCompleteDropDown.PopupOpened += this.AutoCompleteDropDown_PopupOpened;

private void AutoCompleteDropDown_PopupOpened(object sender, EventArgs args)
{
    (this.radTextBoxControl1.TextBoxElement.AutoCompleteDropDown as RadTextBoxAutoCompleteDropDown).MinimumSize = new Size(264, 3*24+20);  
          
}